[Enc] Adding a constructor to a type with field/property initializers that contain lambdas

[tmat]

This is currently a rude edit. To support this scenario the implementation of DefinitionMap.TryCreateVariableSlotAllocator needs to fetch the EnC debug info from an existing constructor to which the existing field initializers are emitted.

See test EditAndContinueClosureTests.InsertConstructorInPresenceOfFieldInitializersWithLambdas